Tire parameter sensing systems for vehicles typically include a plurality of tire-based units and a single vehicle-based unit. Each tire-based unit has an associated tire of the vehicle and is operative to sense at least one parameter of the tire. The sensed parameter(s) may include temperature, pressure, etc. Each tire-based unit is also operative to transmit a parameter signal indicative of the sensed parameter(s) to the vehicle-based unit. The vehicle-based unit is connected to a display. In response to receiving a parameter signal from a tire-based unit, the vehicle-based unit outputs a signal to the display. The display is responsive to the signal for displaying the sensed tire parameter(s).
It is common for the tire-based units of a tire parameter sensing system to be battery powered. Battery powered tire-based units, however, have specific limitations, such as, for example, a limited life, a limited current supply, and a limited operating temperature range. The design of a tire parameter sensing system using battery powered tire-based units must be mindful of these limitations. As a result, it is common for a battery powered tire-based unit to transmit parameter signals only in response to a determination that a sensed parameter is outside of a desired range. For example, if the desired pressure range is 32 to 36 pounds per square inch (“psi”), the battery powered tire-based unit may transmit a parameter signal to the vehicle-based unit only when the sensed tire pressure is determined to be below 32 psi or above 36 psi. By limiting the transmissions of the parameter signal, the battery life of the battery powered tire-based unit may be extended.
In some tire parameter sensing systems, the tire-based units do not include batteries. Tire-based units that do not include batteries receive energy through induction. Devices for producing an electromagnetic field are located adjacent the tires having the tire-based units. The tire-based units include a coil antenna, which forms a portion of a tank circuit that is designed to resonate at the frequency of the electromagnetic field. The resonant frequency of the tank circuit is determined by the following formula:   f  =      1          2      ⁢                          ⁢      π      ⁢              LC            in which, f is the resonant frequency of the tank circuit, L is the equivalent inductance of the tank circuit, and C is the capacitance of the tank circuit.
The amount of voltage induced in a coil antenna is known to be proportional to the number of turns in the coil antenna. Thus, increasing the number of turns in a coil antenna increases the induced voltage in the coil antenna. Increasing the number of turns in the coil antenna, however, also increases the inductance of the coil antenna. Therefore, as the number of turns of the coil antenna is increased for inducing higher voltages, the inductance of the coil antenna also increases. As a result, in order to maintain the same resonant frequency of the tank circuit, the capacitance of the tank circuit must be lowered by the same factor that the inductance of the tank circuit was increased. For example, when the inductance of the tank circuit is doubled as a result of adding additional turns to the coil antenna, the capacitance of the tank circuit must be cut in half to maintain the same resonant frequency for the tank circuit, as is clear from the formula set forth above.
When the capacitance of the tank circuit becomes small, the tank circuit becomes overly sensitive environment changes, such as, for example, temperature. An environmentally sensitive tank circuit is impractical for use in a tire-based unit of a tire parameter sensing system since the environmental conditions experienced within a vehicle tire vary dramatically, particularly, with regard to temperature.